1. Field
The present application relates generally to the operation of wireless communication devices, and more particularly, to methods and apparatus for providing modified time stamps in a communication system.
2. Background
In time delay sensitive applications, such as Voice over Internet Protocol (VoIP), traffic packets typically are generated at a source terminal with regular time intervals (e.g., 20 ms). However, these packets may not arrive at the destination terminal with uniform time intervals. For example, in a mobile to mobile voice call, traffic packets first travel over a reverse link (RL) from the source, then over a core network, and then arrive at a destination radio access network (RAN). Packets usually do not arrive at the destination RAN with uniform time intervals due to one or more jitter sources. For example, traversing a core VoIP network may introduce delay jitter. In a packet switched network, additional delay jitter occurs due to queuing and transmission on the forward link to a destination device. Typically a de-jitter buffer provided at a receiving device observes a combination of these multiple jitter effects.
One technique to reduce the end-to-end jitter is to associate each packet at its source with a real-time timestamp. At the destination, a forward link scheduler uses this timestamp to identify packets that have experienced a long delay so they can be scheduled sooner than packets having a shorter delay. In this way, those packets having experienced a longer delay would experience a relatively short delay on the forward link to compensate for the long delay that has been experienced up to the scheduler.
Notwithstanding, there are several problems associated with the use of real-time timestamps for scheduling packets at the forward link scheduler. First, timing logic at the source needs to be synchronized with the timing logic at the scheduler in order to accurately determine packet delay. While this may be possible using a global timing standard, such as GPS, for mobile to mobile calls, it may not be possible for VoIP calls using land-line VoIP phones, or with mobile VoIP terminals that do not utilize GPS. Second, if the forward link scheduler schedules packets using absolute real-time timestamps associated with source terminals, higher priority will be given to calls which experience long delays. For example, long distance VoIP calls will get higher priority than local VoIP calls. Thus, even local calls may experience long scheduling delays and therefore provide an unsatisfactory user experience.
Therefore, what is needed is a mechanism that operates to reduce the end-to-end jitter in a communication network thereby enabling smaller de-jitter buffers at a receiving device while providing a satisfactory user experience.